Sound Design Workshop: Acting on Impulse

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This impulse in Ableton Live is processed with a delay, two IK Multimedia T-RackS mastering plug-ins and a rack of Ableton Live effects.

Impulses, momentary spikes in amplitude, are used in audio mainly as a tool for testing rooms or programming convolution reverbs. An impulse is as close to a non-sound as you can get and still get a sound. That raises an interesting sound-design possibility: triggering audio effects chains with impulses and relying exclusively on the effects to shape the sound. I'll use Ableton Live's robust effects rack implementation for my examples, but you can apply similar techniques using any DAW's effects sends and inserts (see the screenshot above).

Resonance, Reverberation And Reflections

A logical first step is to create decay using delay reflections or reverb. I started by feeding an impulse to a ping-pong delay with a delay-time setting of 8.8 ms. Short delay times (less than about 50 ms) produce a discernible pitch, and the ping-pong of the delay creates a stereo flutter (see Web Clips 1 and 2).

The rapidly repeated impulses create a pitched sound whose frequency is 1,000 divided by the delay time — approximately 114 Hz in this example. (Conversely, divide the desired frequency into 1,000 to calculate the delay in ms.) You can use that to program a melody by automating the time delay parameter. Depending on the modulation capabilities of your DAW, you might use track automation or a MIDI keyboard to control the delay time and, therefore, the pitch.

Next, I used a few other plug-ins to round out the sound. I started with Live's phaser plug-in and set its frequency to 123 Hz (B2), which is close to the pitch generated by the ping-pong delay (see Web Clip 3). I followed the phaser with Live's distortion effect, Erosion, to add some body (see Web Clip 4). Live lets you use the arrow keys on your keyboard to nudge effects parameters, and that's a great way to find the sweet spot. As an afterthought, I added Live's Chorus effect with feedback and a slow modulation time to create a squeaky pitch shift (see Web Clip 5).

All effects are fair game once you've given the impulse a body. Try using a compressor to soften the attack. Decrease the dry signal of your reverb or delay to reduce the volume of the original impulse.

Creating Patterns, Exploring Space

Exploring a single impulse processed by a delay with a fixed delay time gets you only so far. To produce random patterns of impulses, first create a loop from the original impulse and then play the loop through the delay effect while manually changing the delay time. Try decreasing it to 1 ms and then slowly sweeping it up to a higher value. That will give you a random, rhythmically coherent pattern (see Web Clip 6).

To create something more suggestive, I dropped the pitch of the sample and added Live's Saturator to the track. Both of those serve to thicken the impulse and fatten the sound (see Web Clip 7).

Wondering what a more spatial effect would sound like, I first tried using multiple delays on multiple tracks panned left and right. Eventually, I opted for a natural sound source: Kellogg's Rice Krispies. That's right, maybe the crazy little elves were onto something. I picked a quiet moment to set up a pair of condenser mics in the kitchen and dropped Rice Krispies on several pans (see Web Clip 8). Lowering the pitch of the sample and shaping it with a phase EQ evoked natural scenes like a crackling fire or the sound of rocks churning underwater (see Web Clips 9 and 10).

Feeding an impulse into a delay may sound a bit like using a pulse oscillator, but delay-time modulation and the ability to create repeating syncopations and stereo patterns adds an experimental dimension that is absent in simple synthesis. In addition, using a different impulse or using a recording of an impulse-like sound (such as a Rice Krispies snap) effects predictable timbral changes. In this article, I've used impulses, but you can use any short sound-grain you want.

Benjamin McFarlane is editor-at-large and Web producer of ModernBeats Hit Talk (